“One of the most interesting and difficult of the problems that confront the student of the New Testament must here be dismissed with little more than a word. What is the literary relation between the first three Gospels? How did they come to be so much alike?
“Our first impulse, perhaps, is to say that the Synoptic Gospels are similar because they are concerned with the same things. This explanation, however, is quickly seen to be insufficient. It would explain the agreement in matters of fact. But the similarity between the Gospels extends also to the minutest coincidences of expression. Two trustworthy narrators of the same events, although they will agree in the facts, will, if they are independent of each other, differ widely in expression.
“A second explanation, therefore, suggests itself. Perhaps one of the Gospels was dependent upon one or both of the others. This explanation might serve to explain the similarity between Mark and Matthew and between Mark and Luke. But it utterly fails to explain the similarity between Matthew and Luke in those portions where both of these Gospels have no parallel in Mark. For if one thing is clear, is that Matthew and Luke are quite independent of each another. That is demonstrated, if by nothing else, by a comparison of the infancy narratives at the beginning of these two Gospels.
“In order, therefore, to supplement the theory of dependence of one Gospel upon another, it has been suggested that Matthew and Luke used in addition to Mark another common source which has now been lost. This so-called ‘two-document theory’ has won exceedingly wide acceptance among modern scholars. It is held in many modifications, but the essence of it is that Matthew and Luke had two written sources in common: (a) Mark, and (b) a source containing chiefly discourses of Jesus.
“The detailed evidence for and against the ‘two-document theory’ cannot be here discussed. At least this much, however, must be said: this theory, even if correct—which is by no means certain—is insufficient. It fails to explain the differences between the Gospels which run along with the similarities. If, for example, in the passages where Mark and Matthew are parallel, Matthew was dependent upon Mark and only upon Mark, then it is difficult to explain why he made just the changes that he did in the Marcan text. Some of the changes can no doubt be explained—as due to a desire for brevity or smoothness or the like—but many, if they be merely changes of what Mark wrote, seem to lack both rhyme and reason. What needs to be emphasized, therefore, against the modern one-sided acceptance of the two-document theory is that all of the evangelists stood in the full current of oral tradition. When Luke or Matthew differ from Mark the differences should not be dismissed as mere unauthorized editorial changes, but should be regarded as preserving valuable independent information.”
(J. Gresham Machen, The New Testament: An Introduction to its Literature and History [Carlisle: Banner of Truth, 1990], pages 211-2.)